The present invention relates to the field of satellite communication. More specifically, the present invention relates to the field of satellite communication utilizing on-satellite baseband processing.
High-frequency (e.g., microwave and millimeter-wave) communication typically requires a line-of-sight communication path. When a signal source and a signal destination are located on the surface of the Earth, the curvature of the Earth and other obstructions (terrain, etc.) impose a severe limitation on the line-of sight communication path.
The distance between the signal source and destination may be increased by incorporating one or more repeaters into the communication system. If the repeater is a satellite in orbit about the Earth, the potential distance between the signal source and the signal destination is significantly increased. If the satellite is located in a geosynchronous orbitxe2x80x94approximately 35,786 kilometers above the surface of the Earthxe2x80x94surface distances between signal source and signal destination in excess of forty-five percent of the circumference of the Earth (i.e., in excess of 18,000 kilometers) may be spanned. With the use of three geosynchronous equatorial satellites, therefore, any location on the surface of the Earth, other than in the extreme polar regions, may be reached.
A repeater satellite receives an uplink signal having a given payload data (intelligence), and transmits one or more downlink signals having substantially the same payload data. This is substantially a xe2x80x9cbent-pipexe2x80x9d function, i.e., an upward-propagating signal is bent into a downward-propagating signal by the satellite.
To fulfill a wide variety of customer needs, a satellite-based communication system should be able to effect both point-to-point and point-to-multipoint communication services. Desirably, such a system would also be able to provide signal routing. Under certain conditions, it may be desirable to process the payload data of a given signal within a satellite. An example of this may be when a single uplink signal is to be transmitted as a different downlink signal to each of a plurality of signal destinations. In such a case, the payload data (intelligence) of each downlink signal may be radically different than the payload data of the originating uplink signal. To fulfill this need, the satellite must be able to demodulate the uplink signal into a baseband signal, process the source payload data to produce each destination payload data at the baseband level, and modulate the resultant baseband signals into the requisite downlink signals. Effecting this conversion and processing requires a considerable expenditure of energy.
The energy budget of a satellite is finite. In a xe2x80x9cbent-pipexe2x80x9d operation, a significant portion of the energy budget is consumed in the transmission of the downlink signals, if the satellite is also to perform on-board baseband processing, then a significant portion of the energy budget is consumed effecting that processing. A compromise in the energy budget is therefore in order when a single satellite is to achieve both goals. Such a compromise satellite is exemplified by the NASA Advanced Communications Technology Satellite (ACTS) experiment, wherein the satellite used effected both xe2x80x9cbent-pipexe2x80x9d and processing functions. While the ACTS satellite successfully demonstrated the technical viability of a satellite employing both bent pipe and baseband processing, it did not address the commercial viability of such a communication system.
Assuming for the sake of this discussion that a given satellite is placed in a geosynchronous orbit, then certain compromises on that satellite must be made. Ideally, the satellite will be optimized for each of three critical parameters: energy, mass, and cost.
Energy consumption generates heat. In space, this heat may be extracted only by radiation. For a given technology, as a satellite becomes more complex and performs more functions, the circuitry therein increases. The increased circuitry leads to an increase in the capacity and size of the power source to provide the additional energy. The increase in both the circuitry and the power source leads to an escalating increase in the overall mass of the satellite. Heat is a function of the energy consumption. Heat is therefore, indirectly, a function of the mass of the satellite. Since radiation is a function of surface area, a point is reached where an increase in satellite functionality (and mass) will exceed the satellite""s ability to dissipate the resultant heat.
Mass poses other problems as well. The more massive a satellite is, the greater the cost of orbital insertion. Also, once inserted, the satellite must be maintained in attitude and position. This requires onboard fuel, engines, and control circuitry. As the mass of the satellite increases, its inertia increases. The resultant fuel consumption for maintenance burns therefore increases. Assuming a given satellite life, the overall quantity of fuel also increases, which increases the mass, etc. Again, a practical limit is soon reached.
One solution to the mass problem would be to reduce functionality per satellite, i.e., to use two identical satellites to achieve a given throughput. Another complication with identical satellites is the independent frequency plans on the transmit side. This would require two insertions for a given amount of functionality, thereby significantly increasing the cost. What is most desirable, then, is to use satellites that are optimized for energy, mass, and cost.
What is needed, therefore, is a communication system that serves both bent-pipe and baseband processing functions in an efficient and cost-effective manner.